The present invention relates to a yarn sensor for optically scanning a yarn, moving in its longitudinal direction in a measurement gap.
Optical systems are often employed for contactless detection of yarn parameters with the yarn in motion in a spinning or bobbin winding machine. Optical systems are simple in their construction and function and can be made economically. They operate with shading or with reflected light. The test material is lighted by a light source.
European Patent Disclosure EP 0 761 585 A1 describes a generic type of yarn sensor with an optically functioning system that can likewise serve not only to determine the yarn diameter, but also to detect extraneous material in the yarn, such as extraneous fibers or contaminants. The surfaces in the measurement gap that are struck by light projected by the light source reflect this arriving light. The light reflected by the yarn, because of the small surface area of the yarn, represents a relatively small signal source. The yarn signal converted into current varies in the nanoampere range. Compared to the small irradiated surface of the yarn, the relatively large, dirty surface of the measurement gap, because of its length, represents a not inconsiderable source of reflection signals. The interfering radiation, which adulterates the result of the measurement, is also called a parasitic signal. Because of the low intensity of the yarn signal, a high amplification of the signal converted from the incident light at the yarn takes place, but high amplification of the parasitic signals occurs as well. This leads to an impermissibly small useful signal, in proportion to the total signal. The yarn sensor of European Patent Publication EP 761 585 A1 is incapable of overcoming this disadvantage.
Swiss Patent Disclosure CH 643 060, like European Patent Publication EP 0 761 585 A1, also describes an optical system for checking the yarn diameter. A measurement signal that is proportional to the diameter of the yarn is generated. Signal fluctuations that occur because of changes in the light intensity of the light source as a result of fluctuations in the supplied voltage, aging, or clouding are compensated with the aid of circuitry means. In an exemplary embodiment shown, a point light source emits a cone of light in the direction of a camera or other picture taker, and the yarn passing between the light source and the camera is projected as a shadow on the camera. A CCD line sensor serves as the camera. The extent of the shading on the camera is dependent on the diameter of the yarn. The location of the yarn and in particular the distance of the yarn from the camera has a marked influence on the size of the shaded area. For instance, if the yarn moves toward the camera while the yarn diameter remains constant, the shading becomes smaller, even though the yarn diameter has stayed the same. This leads to adulterations in the outcome of measurement.
In a further alternative exemplary embodiment shown in Swiss Patent Disclosure CH 643 060, an optical element is disposed between the light source, which is said to be of the point type, and the yarn and is intended to cast the light, emitted by the light source, onto the camera in the form of an approximately parallel beam. In this manner, it is intended that the shading and hence the outcome of measurement are no longer affected by the location of the yarn in the measurement region or measurement gap. Motions of the running yarn transversely to the running direction in the measurement region are tolerable in this case. The parallelism of the beam is dependent on whether the light source is ideally point-shaped. However, ideally point-shaped light sources are not available. Even with the incandescent bulbs that Swiss Patent Disclosure CH 643 060 describes and calls point light sources, the light is not generated at a point. Typically, in incandescent bulbs, the light is generated by a mounted incandescent filament. Since the precondition of a point-shaped light source cannot be met, neither the uniformly distributed luminous intensity nor the parallelism of the beam aimed at the camera can be attained, and remain inadequate. An outcome of measurement that is independent of the location of the yarn in the measurement gap is unattainable with the apparatus of Swiss Patent Disclosure CH 643 060.
Another possible way of purposefully generating parallel light is to convert the light emitted by a Lambert emitter. The light density of a Lambert emitter is constant in all directions of the half-space defined by the light-emitting face. That is, the Lambert emitter behaves like an ideal diffusely emitting face. Examples of widely used surface emitters of this kind are fluorescent lamps, but because of the required structural size, they are unsuitable for use in yarn measurement heads at the work stations of a spinning or bobbin winding machine. Light-emitting diodes are also area emitters, and as a rule have the attribute of having a Lambert conformal emission characteristic. By definition, the radiation emitted by every point in the surface of a Lambert emitter is embodied as a divergent beam of light.
German Patent Disclosures DE 23 37 413 B2 and DE 198 59 274 A1 show devices for monitoring a traveling yarn, in which light-emitting diodes are used as light sources, and photodiodes are used as light receivers.